1. Field of the Invention
The present invention relates generally to membrane filtration systems and in particular to a membrane sampling device that provides indicia concerning the performance of the membrane and the operating conditions in a pressure vessel used in ultrafiltration (UF) or reverse osmosis (RO) systems.
2. Discussion of the Prior Art
Conventional membrane filtration systems can be used in various fields for applications to filter soluble salts or suspended particles from a feed liquid, for example, RO applications in filtering seawater or brackish water. Such membrane filtration systems typically use a pressure vessel to house one or more membrane elements that filter the feed liquid. Conventional membrane elements are typically manufactured in diameters ranging from approximately four to eight inches and in lengths of forty to sixty inches and, as a result, each membrane element can have approximately four hundred square feet of membrane area. Multiple pressure vessels can be configured in groups or concentrate stages, whereby the concentrate from the first stage is supplied to the feed of the second stage and so on for other concentrate stages. The parameters of flow and pressure in the pressure vessel are controlled with feed and concentrate valves, whereby the feed valve is disposed after a high pressure pump to control the flow of feed to the pressure vessels of the concentrate stage and the concentrate valve is disposed at the outlet of the concentrate stage to control the feed pressure thereto. In this manner, each concentrate stage filters the soluble salts or suspended particles from the feed liquid.
Membrane filtration systems can have problems of compaction and fouling that diminish the performance of such systems. For example, membrane material exposed to a feed flow at high pressure or high temperature will increase the density of the membrane material, which is referred to as compaction. Such compaction lowers membrane performance by decreasing the flux or rate of diffusion of water and dissolved constituents through the membrane material, whereby higher feed pressure has to be applied to maintain a desired permeate flow. Compaction also lowers salt diffusion through the membrane material, thereby resulting in lower permeate salinity.
In addition, membrane fouling has a negative effect on membrane performance and, in extreme cases, may result in non-reversible membrane degradation. Membrane fouling is caused by deposits of inorganic or organic substances on the membrane surface and/or blockage of the feed channels formed in the membrane elements. In the initial stages of membrane fouling, a decrease in the system performance is characterized by an increase in the pressure drop across the membrane filtration system. If such membrane fouling continues uncontrolled, membrane performance will continue to decrease until the performance of the system falls below a useful level for a given application, whereby membrane elements have to be replaced at such point. Conventional tests to determine the nature and extent of fouling require that a sample of the membrane element be taken and analyzed in order to evaluate the condition of the membrane element and to characterize the membrane surface. Such tests require opening the pressure vessel to remove and take samples of the membrane elements, thereby stopping the normal operation of the membrane filtration system. Membrane fouling problems are particularly costly in membrane filtration systems that pack one or more membrane elements into a particular pressure vessel because extensive time is required to initially pack and then unpack such membrane elements from the affected pressure vessels.
Conventional membrane filtration systems have not addressed the above-identified problems and such problems are not remedied in the prior art. Both compaction and fouling problems are characterized by a need to increase feed pressure to produce design permeate flow. Fouling by suspended particles also may result in increase of pressure drop in the system. Determining the condition of membrane elements requires removal so as to conduct a desired test and, additionally, such tests may require the use of dedicated equipment or a particular kind of testing facility. If the desired test has to be conducted at a special testing facility, replacement membrane elements are required for continued system use, thereby increasing the cost, labor expended, and downtime of the system. For the purpose of identifying membrane fouling and providing an effective remedy, it is important to obtain information on performance of individual membrane elements. Such information is not available in the current membrane filtration systems.
The membrane sampling device of the present invention seeks to overcome these and other shortcomings of the prior art so as to obtain information on the performance of the membrane filtration system in an effective and convenient way.